Apparatus for improving the torsional fatigue strength of crankshafts

ABSTRACT

Crankshafts frequently have low torsional fatigue strength in the region of the oilholes on the main bearing surfaces when inductively hardened. The invention provides a method and apparatus for producing hardened zones which increase in depth along the main bearing surface towards the oilhole, which oilhole is preferably hardened along its whole length, whereby the torsional fatigue strength is improved.

United States Patent Inventors Appl. No. Filed Patented Assignee APPARATUS FOR IMPROVING THE TORSIONAL FATIGUE STRENGTH OF CRANKSHAFTS 4 Claims, 2 Drawing Figs.

11.8. C1 219/ 10.57, 219/1043, 219/1059 Int. Cl 1105b 5/00, H05b 9/06 Field of Search 219/1059, 10.57, 10.43

[56] References Cited UNITED STATES PATENTS 2,293,047 8/1942 Denneen et a1 219/1057 X 2,761,048 8/1956 Tudbury 219/1057 X 2,800,809 7/1957 Pike 219/1043 3,272,954 9/1966 Seulen et a1. 219/1043 3,510,928 5/1970 Seulen et a1. 29/1495 Primary Examiner-J. V. Truhe Assistant ExaminerL. H. Bender AttorneyCushman,'Darby & Cushman PATENTEUunv 23 IQH SHEET 1 OF 2 i J I Fly. 2

In an 01's PATENTEDuuv 23 19?! 3,623,128 SHEET 2 [IF 2 fig- Fly. 4

Z In en ors APPARATUS FOR IMPROVING THE TORSIONAL FATIGUE STRENGTH OF CRANKSIIAI'TS This application is a division of our copending application Ser. No. 704,89l,filed Feb. 12, 1968.

This invention relates to the inductive hardening of crankshafts, and particularly relates to the improvement of the torsional fatigue strength of inductively hardened crankshafts.

For improving the wear resistance of crankshafts it has previously been the practice to provide the bearing surfaces thereof, i.e., the crankpins and main bearing surfaces, with hardened layer produced almost exclusively by an inductive hardening process.

Inductively hardened crankshafts have been proposed in which the hardened layers are not confined to the surface principally exposed to abrasive wear, but which also extend into the angles between the bearing surface and the crank webs, and further into the axial abutment faces. Such hardened layers impart a considerable degree of bending fatigue strength to crankshafts thus treated.

Crankshafts which have been inductively hardened in this way have proved to be very satisfactory in practice. By thus hardening crankshafts without modifying them in any other way it has been possible to use them in engines developing much higher power than would have been possible when ordinarily hardened crankshafts are used. The hardening of the angles at the webs is capable of improving the bending fatigue strength of crankshafts by about 80 to I percent.

It has now been found that when overloads arise in service, the most critical part of a crankshaft that has this improved bending fatigue strength is the region surrounding the oilholes in the main bearings and crankpins. If the static and dynamic loads exceed a critical level torsional fracture first occurs at these points, possibly because of the reduced volume of material left in the region of the oilholes.

So far this tendency to failure of crankshafts has not successfully been countered. Attempts have been made to eliminate the causes of torsional fatigue fracture by improving the surface quality at the edges of the oilholes or by subsequent work hardening. However the expense of carrying out this method in mass production for achieving the desired results, is too high. Attempts at solving the problem by using high-alloyed steels have also failed because the cost of the product becomes uneconomically high due to the higher cost of materials and expensive machining.

The object of the invention is to improve the torsional fatigue strength of low-alloyed or unalloyed steel crankshafts which have been inductively hardened to improve the torsional fatigue strength thereof.

The invention consists of a method of improving the torsional fatigue strength of crankshafts having an oilhole in the main bearing and/or crankpin comprising inductively heating and bearing surface extending at least from the angle between the bearing surface and the crankweb to provide an inductively hardened surface layer which progressively increases in depth towards the said oilhole.

According to a preferred feature of the invention, the depth of the surface hardened layer increases in depth towards the oilhole in such a way that the inside surface of the entire length of the oilhole is provided with a hardened surface layer. It may be preferred however inductively to harden the interior of the oilhole to a distance of about half the diameter of the shaft or to a distance of less than half the diameter of the bearing surface.

For performing the method according to the invention apparatus may be used comprising two inductors which take effect consecutively during the hardening of the crankshaft. Both heating conductor loops or inductors are independently controllable with respect to power, frequency and heating time.

Embodiments of the invention are hereinafter described and illustrated in FIGS. 1 to 4 of the accompanying drawings, of which:

FIGS. 1 and 2 each show the crankpin of a crankshaft that has been provided with hardened layers of different depths.

FIG. 3 is a schematic representation of apparatus for carrying out the method according to the invention, and

FIG. 4 is a representation in graphic form of the electrical power supplied to the inductors of the apparatus of FIG. 3, against time.

Referring to FIG. I, a pin I and a crank webs part of a crankshaft includes a crank- 2. On each side of the crank the shaft extends to form main bearing surfaces 3. Each bearing, i.e., the crankpin and the main bearing of the shaft, is provided with oilholes 4 in the form of bores, which may extend diametrically as shown in FIG. 1 or at an angle, through the bearing.

The boundary between the inductively hardened and unhardened material is indicated in FIG. 1 by a discontinuous line, the zone of hardening being indicated by 5. The hardened zone 5 at each end of the crankpin in the abutment zone embraces the angle between the pin and the web and then progressively increases in thickness towards the oilhole 4. The hardened zone 5 in FIG. I on the surface of the crankpin completely penetrates the crankpin in the neighborhood of the oilhole. Consequently the entire inside surface of the oilhole is surface-hardened.

FIG. 2 shows an alternative degree of hardening, the zone of hardening being likewise indicated at 5, which progressively increases in depth towards the oilhole 4, but in the neighborhood of the oilhole the depth of penetration into the crankpin is not so great as that shown-in FIG. 1. The internal surface of the oilhole is not therefore surface-hardened throughout its entire length. Apparatus according to the invention for improving the torsional fatigue strength of a crankshaft'having an oilhole in a main bearing portion and/or a crankpin portion of the shaft for the lubrication of the bearing surface thereof, comprises two inductor elements adapted to produce heating zones of different width in the direction of the axis of the said main bearing portion, and means for consecutively energizing the said inductor elements for hardening the crankshaft.

One embodiment of apparatus for carrying out the method of hardening according to the invention is schematically shown in FIG. 3. Two inductors or two heating conductor loops of one inductor, are located above the surface 6 that is to be hardened, the surface being shown developed in the plane of the paper. The heating zones produced by heating conductor loops or inductors 7 and 8 are different in width, due to their shape and configuration. The elements 7 and 8 may be described as inductors if they are mechanically separate. However, heating conductors which are supplied with different electrical powers, at difierent frequencies at different times may very well be contained in one inductor casing. In the following description it is therefore assumed for the sake of simplicity that 7 and 8 are merely heating conductor loops, although two separate inductors might just as well be used.

Thus the two heating conductor loops 7 and 8 are each connected to an electrical medium frequency generator through a switch 11 and 12 respectively, However, normally the heating conductor loops are connected to different electrical generators which may differ more particularly in the frequency they supply. The said inductors are placed adjacent the portion of the crankshaft to be heated, and the crankshaft rotated so that relative movement occurs between the crankshaft and the conductor loops 7 and 8.

The region surrounding the oilhole 4 is heated and after the surface has reached hardening temperature heating is continued to maintain the same temperature level until a heating zone of the desired depth has been produced. Isothermal heating may partly be provided by the second inductor although the principal purpose of this inductor is to heat up the entire bearing surface, possibly including the angles at the webs. The heating times are so displaced that the workpiece can be quenched from a common temperature level.

The process of inductive heating of the surface that is to be hardened is intended to be so performed that the first heating conductor loop 7 is first energized by the generator G at lower power N for a period as indicated in the graph in FIG. 4, in

which the ordinate represents power N supplied to the coils and the abscissa represents time t. At the end of the time t, the switch 11 is opened and the switch [2 closed. The heating conductor loop 8 is now supplied by the generator G with a power N for a period 1,. If the power. frequency, time and shape of the heating conductor loops are correctly choosen, the bearing surface will have been inductively heated at the end of the total period +1 in the manner illustrated in FIG. 1 or FIG. 2 so that hardening can be accomplished by quenching with a liquid or gaseous medium.

Satisfactory results will be achieved if the heating conductor loop 7 is supplied with electrical energy of lower frequency. It is nevertheless possible to supply both heating conductor loops from the same electrical generator, particularly as a result of the recent availability of high power inverters using thyristors whereby the frequency of the supply voltage can be more easily adjusted within limits to different values, compared to the use of conventional static or dynamic mediumfrequency generators.

What is claimed is:

1. Apparatus for improving the torsional fatigue strength of a crankshaft having a main bearing portion and a crankpin portion having a bearing surface comprising two inductor elements for heating respectively overlapping areas of said bearing surface having different widths in the direction of the axis of said main bearing portion, and means for consecutively energizing the said inductor elements for hardening the crankshaft.

2. Apparatus as in claim 4 wherein said elements are inductor loops.

3. Apparatus for improving the torsional fatigue strength of a crankshaft having an oilhole, a main bearing portion and a crankpin portion having a bearing surface comprising two inductor elements for heating respectively overlapping areas of said bearing surface having different widths in the direction of the axis of said main bearing portion, means for rotating the said main bearing portion relatively to the said inductor elements, and means for energizing the said inductor elements whereby a first heating zone is produced in the region of the oilhole. and a second overlapping heating zone is produced extending at least between the angles between the said main bearing surface and the crankwebs, the combination of the said first and second heating zones producing a heated zone which when quenched produces a hardened layer which increases in depth towards the oilhole.

4. Apparatus according to claim 3, comprising means for independently controlling the power, frequency and heating time to the said inductor elements.

i t i i i 

1. Apparatus for improving the torsional fatigue strength of a crankshaft having a main bearing portion and a crankpin portion having a bearing surface comprising two inductor elements for heating respectively overlapping areas of said bearing surface having different widths in the direction of the axis of said main bearing portion, and means for consecutively energizing the said inductor elements for hardening the crankshaft.
 2. Apparatus as in claim 4 wherein said elements are inductor loops.
 3. Apparatus for improving the torsional fatigue strength of a crankshaft having an oilhole, a main bearing portion and a crankpin portion having a bearing surface comprising two inductor elements for heating respectively overlapping areas of said bearing surface having different widths in the direction of the axis of said main bearing portion, means for rotating the said main bearing portion relatively to the said inductor elements, and means for energizing the said inductor elements whereby a first heating zone is produced in the region of the oilhole, and a second overlapping heating zone is produced extending at least between the angles between the said main bearing surface and the crankwebs, the combination of the said first and second heating zones producing a heated zone which when quenched produces a hardened layer which increases in depth towards the oiLhole.
 4. Apparatus according to claim 3, comprising means for independently controlling the power, frequency and heating time to the said inductor elements. 